The issue of non-assigned seating is one of the most loathed facets of the no-frills airline industry, but I think it's important that we weigh up both sides of the argument. Southwest, the self-proclaimed granddaddy of no-frills airlines, said they originally didn't allocate seats because they used to only have a few people booked on their planes so assigned seating was a bit of a joke, but as their flights got busier they realised it was actually a way to get people onto the aircraft faster, because people tended to show up to the gate ontime to ensure they got the seats they wanted. Crews on Southwest, generally known for being extroverts & more than happy to use the PA at any opportunity, tend to stand at the front of the cabin and ask passengers "to keep moving on down" the aircraft to fill up seats as soon as possible. This procedure is one of the things that has helped Southwest to remain consistently profitable for over thirty years, and to achieve their 20-minute turnarounds, another issue debated on this forum.
When bmibaby started, we mirrored a service feature incorporated by Go. Passengers found the experience much less stressful to have an allocated seat, and whilst it took slightly longer to check people in, there was no real difference in boarding time. In fact, it was just as fast because people knew exactly where they were sitting, so could head straight to their seats with the encouragement of their crews.
The issue of airbridges tends to stem from the idea that no-frills airlines want to keep costs down by not using these stands as they can be more expensive at some airports, but also to allow both sets of steps to be used. I agree that the use of an airbridge is better, not only from the point they are more comfortable, but also because it is faster to board those with special needs such as pax with babies or those with limited mobility or disabilities. The issue has to be that the airbridge is attached & secured as quickly as possible, which comes with good training, and is indeed utilised by Southwest, the steps thing being a Ryanairism, and is also used by most no-frills airlines on the continent.
Either way, there are certainly reasons for & against, and it's certainly worth bearing in mind that the no-frills airlines have done a fantastic job in revolutionising travel for us, making it just like a bus, and non-assigned seating is an aspect of this. However, I stick by the idea that pax are often willing to pay for an assigned seat, and if the price was identical between an airline with & without assigned seats, most would chose the former, it can be that important in the decision process. bmibaby, thomsonfly and the charters, have used this to their advantage by charging people to pre-allocate seats, something I could potentially see easyJet doing.